Considerable effort and advanced technology is being applied to recovering a maximum amount of oil from subsurface formations. In even the most permeable reservoirs, a residual quantity of oil remains on the inorganic matrix of the reservoir after all of the recoverable oil is removed. Reservoirs that contain very heavy and viscous oil, including oil that will not flow at any reasonable recoverable rate at the temperature and pressure of the reservoir, retain even more oil after the easily recoverable material is produced. Tight shales, oil shales, and other largely impermeable rock formations are also difficult to produce. Considering the rapidly decreasing amount of easily accessible reservoir hydrocarbons on earth today, it remains a growing challenge to access and produce hydrocarbons that were not previously available for commercial production.
Improved oil recovery methods, including water flooding, chemically enhanced, and thermally enhanced oil recovery (TEOR), such as by steam flooding, have greatly expanded the number of known oil-containing reservoirs that may be produced in commercial quantities. Enhanced oil recovery methods involving solvents other than water, some with carefully designed surfactants for dislodging residual oil from clay granules in the inorganic matrix of the reservoir, are even more effective for removing remaining traces of oil from reservoirs.
One disadvantage of the more complex steam and chemically enhanced methods is the cost of materials and processing in utilizing these methods. A second disadvantage with all of these improved recovery methods is the limited extent to which they can actively improve the transport properties of oil from formations that have low permeability. Each requires providing fluid (liquid, gas or vapor) access to the formation in order to introduce the chemicals or steam that serve to improve the transport properties of the oil in the formation. Thermal methods, as well, depend largely on heat conduction through a fluid-filled rock matrix. Particularly in a rock matrix having low permeability, such thermal methods are inefficient for improving the transport properties of oil from the matrix. Radiation of energy via radio frequency and microwave frequency from an antenna in a wellbore has been studied for many years (see, for example, U.S. Pat. No. 5,293,936). But, applying antenna designs that were developed for communication in air to subsurface reservoir heating design has proven to be difficult. Most antennas are designed to operate in a low-loss environment having low relative electrical permittivity and little or no electrical conductivity, such as in the Earth's atmosphere.
The typical considerations applied to antenna designs that are intended to be used in a low-loss environment such as free space do not apply to antennas intended to be used in a lossy environment such as underground. In other words, an antenna designed to operate in free space will operate very differently when placed in a lossy environment. Therefore, there is a need for antennas specifically designed to operate within the lossy environment in order to achieve the desired performance characteristics.
Antennas designed to operate in free space (or in a terrestrial based system in air) are typically designed to achieve a desired far field radiation pattern to accomplish communication goals (radio) or for detection purposes (radar). The primary design considerations are often directed to obtaining a desirable operational bandwidth, impedance characteristics, as well as directionality of radiated energy (expressed by far field radiation pattern). Penetration depth (the distance over which electric field of a plane wave is reduced to 1/e of its initial value) in air is hundreds, or thousands or millions (and more) of times the wavelength of the propagating wave.
Numerous investigators have published research results on using electromagnetic methods for enhanced oil recovery. However, the application of electromagnetic methods to subsurface formations has generally been plagued by the development of uneven heating of the wellbore and formation rock immediately adjacent the wellbore. Some attention has been paid to the problem of non-uniform heating of formation rock using electromagnetic methods. For example, Bridges, in U.S. Pat. No. 5,293,936 attempted to resolve the uneven heating problem when using a monopole or dipole antenna-like apparatus by modifying edge and power input regions to purportedly achieve equal distribution of electric fields. More recently, Kinzer, in US20070108202A1, suggested switching out different electrode element pairs for moments of time or possibly providing different field strengths to different portions of the formation or stratification to achieve more uniform heating of the formation.
There is still a need for improved antenna design to meet the challenges of heating lossy environments using electromagnetic waves.